1. Field of the Invention
The present invention relates to a display device, a display panel driver, and a backlight drive method, and especially relates to a brightness control of backlight of the display device.
2. Description of the Related Art
One problem in recent LCD (liquid crystal display) devices, specifically LCD devices mounted on portable terminals, is increased power consumption. As the LCD panel has grown in size with improved resolution, the power consumption of the liquid crystal display increases year by year. The backlight is a large consumer of power, and accordingly reduction of the power consumption of the backlight is an effective means for reduction of the total power consumption of the liquid crystal display.
Optimization of brightness control of the backlight is one effective approach to reduce the power consumption of the backlight without deteriorating the image quality. Reduction of the backlight brightness in displaying a dark image effectively reduces the power consumption without deterioration the image quality. Meanwhile, the power consumption can be reduced by lowering the backlight brightness when the liquid crystal display device is used in a dark environment. In addition, recent cellular phones have a function of setting the backlight brightness in accordance with a user setting, which suppresses the power consumption of the backlight. For example, Japanese Laid Open Patent Application No. P2005-148708A discloses a technique to control the backlight brightness on the basis of the picture level of the display image, that is, values of image data of each frame. Furthermore, Japanese Laid Open Patent Application No. 2003-161926 discloses a technique to control the backlight brightness on the basis of the lightness of a use environment (that is, environment light intensity).
One issue in optimization of the brightness control of the backlight is limitation of the number of adjustment steps of the backlight brightness. In commonly-used backlight drive circuits (for example, LED drivers for LED (Light Emitting Diode) backlights), the number of adjustment steps of the backlight brightness is specified in the specification. More specifically, a PWM signal whose duty ratio is variable in 256 steps is generated by a brightness control circuit, and the backlight drive circuit is configured to drive the backlight in brightness in accordance with the duty ratio of the generated PWM signal. Accordingly, the brightness control circuit is configured to control the backlight brightness by 8-bit data. However, the adjustment in 256 steps may be insufficient for a specific control algorithm of the backlight brightness.
Let us consider the following control algorithm as one example:
(1) 8-bit backlight brightness data are used to specify the backlight brightness; and
(2) the allowed range of the backlight brightness data is determined depending on the environment light intensity, and the backlight brightness data are determined within the determined range on the basis of the picture level.
When the environment light intensity is high (that is, when the use environment is bright), the maximum backlight brightness is increased and the allowed range of the backlight brightness data is widened accordingly. On the other hand, when the environment light intensity is low (when the use environment is dark), the maximum backlight brightness needs to be reduced and thus the allowed range of the backlight brightness data is narrowed. This undesirably leads to reduction in the number of adjustment steps of the backlight brightness for the dark use environment.
When the number of adjustment steps of the backlight brightness, which depends on the environment light intensity, is reduced below the number of steps necessary for providing desired picture contrast, the backlight brightness is not suitably controlled on the picture level. When the picture level is represented by 6-bit data, for example, the number of adjustment steps of the backlight brightness is required to be at least 64. However, when the number of adjustment steps of the backlight brightness, which depends on the environment light intensity, becomes smaller than 64, the backlight brightness may be improperly controlled based on the picture level.
To handle such problem, the backlight drive circuit and the brightness control circuit for controlling the drive circuit need to be configured so that the backlight brightness can be adjusted in more steps. When the backlight brightness can be adjusted in many steps, the shortage of the number of adjustment steps of the backlight brightness does not occur.
One approach may be to configure the brightness control circuit so as to generate the PWM signal whose duty ratio can be changed in many steps (for example, 1024 steps), and to configure the backlight drive circuit so as to be adapted the PWM signal that is variable in many steps. However, such approach undesirably increases the circuit size. In order to generate the PWM signal whose duty ratio is variable with many steps, the brightness control circuit needs to be configured to handle multi-bit control data. This increases the circuit size of the brightness control circuit. In addition, the circuit size of the backlight drive circuit is undesirably increased when the backlight drive circuit is configured so as to process the PWM signal that is variable in multiple steps.